In analytical chemistry, electrophoresis refers to methods that use electric fields to manipulate and separate charged molecules in solution. Capillary electrophoresis (CE) refers to a family of “traditional” techniques where the sample is loaded into a capillary and its components are separated into bands along the axial direction. The separation arises whenever components of the sample have unique electrophoretic mobility. These bands can be detected, for example, by a fixed optical system as they flow past. The apparatus may be relatively simple since high voltage electrodes at each end of the capillary serve to not only separate the sample but also to inject the sample into the capillary by taking advantage of electroosmotic flow. The art of CE involves identifying appropriate parameters, buffers, and additives for each specific analyte to maximize separation and resolution between bands
The limits of resolution in CE are partly determined by diffusion of separated analyte into the surrounding buffer, which broadens the bands. Two methods of countering this effect are called focusing and stacking. The object of stacking is to compress the sample molecules into a narrow band, prior to performing the separation, so that differences in mobility are made more distinct. One kind of stacking is achieved by using electrophoresis to drive molecules against an interface between two solutions of differing conductivity. If the sample is in a low-conductivity buffer, electric field strength across it will be high, and electrophoretic velocity will be high. When the sample ions reach a transition to a high-conductivity buffer, electric field strength will drop, as will their velocity, and they will collect at the interface. However, this technique is limited to batch processing. A need exists in the art for a method and apparatus for continuously separating or concentrating molecules using stacking. So-called “free flow electrophoresis” is another technique known in the art that permits continuous separations, but it is not conventionally used with stacking.